The present invention relates to a pattern formation method for use in semiconductor fabrication and an exposure system employing the pattern formation method.
In accordance with the increased degree of integration of semiconductor integrated circuits and downsizing of semiconductor devices, there are increasing demands for further rapid development of lithography technique. Currently, pattern formation is carried out through photolithography using exposing light of a mercury lamp, KrF excimer laser, ArF excimer laser or the like, and use of F2 laser lasing at a shorter wavelength is being examined. However, since there remain a large number of problems in exposure systems and resist materials, photolithography using exposing light of a shorter wavelength has not been put to practical use.
In these circumstances, immersion lithography has recently been proposed for realizing further refinement of patterns by using conventional exposing light (M. Switkes and M. Rothschild, “Immersion lithography at 157 nm”, J. Vac. Sci. Technol., B19, 2353 (2001)).
In the immersion lithography, a region in an exposure system sandwiched between a projection lens and a resist film formed on a wafer is filled with a solution having a refractive index n, and therefore, the NA (numerical aperture) of the exposure system has a value n·NA. As a result, the resolution of the resist film can be improved.
Now, a conventional pattern formation method using the immersion lithography will be described with reference to FIGS. 7A through 7D.
First, a chemically amplified resist material having the following composition is prepared:
Base polymer: poly((norbornene-5-methylene-t-2gbutylcarboxylate) − (maleic anhydride)) (wherein norbornene-5-methylene-t-butylcarboxylate:maleic anhydride =50 mol %:50 mol %)Acid generator: trifluorosulfonium triflate0.04gSolvent: propylene glycol monomethyl ether acetate20g
Next, as shown in FIG. 7A, the aforementioned chemically amplified resist material is applied on a substrate 1 so as to form a resist film 2 with a thickness of 0.35 μm.
Then, as shown in FIG. 7B, while supplying water 3 onto the resist film 2, pattern exposure is carried out by irradiating the resist film 2 with exposing light 4 of ArF excimer laser with NA of 0.65 through a mask 5. Although a projection lens for projecting the exposing light 4 having passed through the mask 5 on the surface of the resist film 2 is not shown in FIG. 7B, a region sandwiched between the projection lens and the resist film 2 is filled with the water 3. Thus, an exposed portion 2a of the resist film 2 becomes soluble in an alkaline developer because an acid is generated from the acid generator therein while an unexposed portion 2b of the resist film 2 remains insoluble in an alkaline developer because no acid is generated from the acid generator therein.
After the pattern exposure, as shown in FIG. 7C, the resist film 2 is baked with a hot plate at a temperature of 110° C. for 60 seconds, and the resultant resist film is developed with a 2.38 wt % tetramethylammonium hydroxide developer (alkaline developer). In this manner, a resist pattern 6 made of the unexposed portion 2b of the resist film 2 can be obtained as shown in FIG. 7D.
However, as shown in FIG. 7D, the resist pattern 6 formed by the conventional method is in a defective shape.
The present inventors have examined the reason why the resist pattern 6 formed by the conventional method is in a defective shape, resulting in finding the following: When a stage of the exposure system for holding the substrate 1 moves, foams are formed within the solution 3, and the foams scatter the exposing light 4. As a result, the exposing light 4 reaches the unexposed portion 2b of the resist film 2 or the exposed portion 2a of the resist film 2 is excessively exposed. This causes the defect in the shape of the resist pattern 6.
When a resist pattern in such a defective shape is used for etching a target film, the resultant pattern is also in a defective shape, which disadvantageously lowers the productivity and the yield in the fabrication process for semiconductor devices.